The invention relates to a hollow camshaft as well as a process for producing hollow camshafts by employing internal high pressure shaping of hollow pipes.
The invention in particular relates to processes employing pre-shaping in the course of internal high pressure shaping for designing complete hollow shafts from pipe sections, wherein pipe sections are here understood to be arbitrary long hollow bodies, i.e. also square pipes, hexagonal pipes or other hollow profiles.
In comparison with solid hollow shafts, hollow camshafts, such as schematically represented in FIG. 1, offer a weight advantage of up to 75% over shafts of solid material. In comparison to this, constructed camshafts such as known from DE-37 04 092, EP 278292, EP 278292, DE-3428372 or EP 290758 or EP 303845, only offer a weight advantage of 40% in comparison with solid shafts. In addition, constructed camshafts remain to be comparatively expensive to produce, because separate cams and hollow pipes must be stocked and then placed together into a die in a suitable manner. Thus, expensive individual components must be kept in stock and combined.
For this reason hollow camshafts, although their constructed shape has been described theoretically and also in patent applications, have not yet been used in motor vehicles up to now.
The reason for this lies in a suitable process technique for producing these shafts. Suggestions for producing constructed hollow camshafts are recited, for example, in DE-C-19 10 517. These process techniques are, however, completely unusable for producing hollow camshafts from pipe sections. The required degrees of shaping can neither be achieved by electromagnetic nor by electro-hydraulic shaping. Swaging or round kneading as described in DE-C-37 36 453 also has not meet the goal. This type of producing hollow camshafts fails particularly because of the one-sided mass distribution in the cams. The one-step internal high pressure shaping by means of a tool in accordance with U.S. Pat. No. 2,892,254 also does not meet the goal. Although axial feeding of material is provided with this mode of operation, filling the die at the cam tips is not achieved and a considerable stretching of the walls of the cams results. FIG. 2 shows the undesired stretching of the walls on a cam which was produced by means of the known one-step internal high pressure shaping method.
The internal high pressure method or IHV method is here understood to mean the method which was described for example in the Industrieanzeiger Industrial Journal! No. 20 of Mar. 9, 1984, or in "Materialumformtechik" Material Shaping Technology!, vol. 1D/91, pp. 15 et seq.: A. Ebbinghaus "Prazisionswerkstucke in Leichtbauweise, hergestellt durch Innenhochdruckumformen" Precision Workpieces in Lightweight Construction, Produced by Internal High Pressure Shaping!, or in "Werkstoff und Betrieb" Material and Factory! 123 (1990), 3, pp. 241 to 243: A. Ebbinghaus "Wirtschaftliches Konstruieren mit innenhochdruckgeformten Prazisionswerkstucken" Economical Construction by Means of Precision Workpieces Produced by Internal High Pressure Shaping!, or "Werkstoff und Betrieb" 122, (1991), 11, (1989), pp. 933 to 938. To avoid repetition, reference is made in full to their disclosure in what follows.
These processes have been used up to now for example for producing flanges, as described in EP-2395052, or for producing constructed hollow camshafts for fastening cams on a pipe for producing hollow camshafts.